How to do a WiFi site survey, step by step

A predictive design tells you what the network should do. A site survey tells you what it actually does. Walls are never exactly the material you assumed, furniture arrives, and somebody installs an access point upside down. Surveying is how you find out before your users do.

Three kinds of survey

• Passive survey. The device just listens to the beacons every access point broadcasts and records the signal of each network at each point. It shows coverage, signal and co-channel neighbors. It is the most common survey and the one Ondalux Heatmap performs.
• Active survey. The device stays connected to the network while you walk, measuring real performance: throughput, latency, packet loss and roaming between access points. It answers "does it work?" rather than "is there signal?".
• Pre-deployment survey ("AP on a stick"). Before installing, a temporary access point is placed where the design says and measured. It is the best way to validate a design in a building with unknown walls.

What you need

• A calibrated floor plan. The scale must be right, or every point you record will be in the wrong place.
• One laptop for the whole survey. Different WiFi adapters report different signal levels for the same network — differences of 5 dB or more are common. Never mix devices in one survey.
• Time. Each scan takes a couple of seconds, and you will record dozens of points. Plan the route before starting.

Walking the site

1. Plan a route that covers every room you care about, the corridors that connect them and the edges of the coverage area. Corners and rooms behind thick walls matter more than open areas near access points.
2. Stop at each point. Click your exact position on the plan and stay still until the scan finishes. Moving while scanning smears the result over several meters.
3. Hold the laptop the same way. Keep it at the same height (about desk or chest height) and orientation throughout. Your body blocks part of the signal, so be consistent.
4. Space the points every few meters in rooms and along corridors, closer together where the signal changes quickly (behind walls, near doors).
5. Survey in normal conditions. Doors open or closed as they usually are; for a validation survey, do it when the furniture is in place.

Reading the results

Between the points you recorded, the signal is estimated by interpolation. Ondalux Heatmap offers two methods: IDW (inverse distance weighting), simple and predictable, and kriging, which learns from the data how quickly the signal changes with distance. Areas far from any point are left blank on purpose: a measured map should not invent values where nobody measured.

Check the same requirements as in the design: minimum signal and SNR where users work, overlap where they roam, and how many networks on the same channel are heard.

Comparing with the prediction

The most useful step is to compare the measured map with the predictive design. The difference map shows measured minus predicted at each point:

• Consistently negative behind certain walls: those walls lose more than assumed. Increase their attenuation.
• Negative near one access point only: check that access point — power, channel, mounting or a cable problem.
• Positive everywhere: the prediction is conservative, so the design has margin.

After a few corrections, the model matches the building, and future changes (moving or adding access points) can be predicted with much more confidence.

Common mistakes

• Not calibrating the plan scale before starting.
• Changing laptops halfway through, or surveying with a phone and comparing with a laptop.
• Walking while the scan runs.
• Surveying only the open areas and skipping the rooms where users actually complain.
• Judging the network by a single band when clients use another.

Surveying with Ondalux Heatmap

Open the Survey panel, choose your computer's WiFi adapter, press Start measuring and click your position at each stop. You can also import and export survey points as CSV. Currently, scanning is supported on Windows; macOS and Linux support is in development.